The Rugosa, also called the Tetracoralla, are an extinct order of coral that were abundant in Middle Ordovician to Late Permian seas.
Solitary rugosans (eg Caninia, Lophophyllidium, Neozaphrentis, Streptelasma) are often referred to as horn corals because of a unique horn-shaped chamber with a wrinkled, or rugose, wall. Some solitary rugosans reached nearly a meter in length. However, some species of rugose corals could form large colonies (eg Lithostrotion). When radiating septa were present, they were usually in multiples of four.
Rugose corals were found from the Mississippian period (345 million yers ago) and have a skeleton made of calcite that is often fossilized. Like modern corals, rugose corals were invariably benthic, living on the sea floor or in a reef-framework. Although there is no direct proof it is inferred that these Palaeozoic corals were provided with stinging cells to capture prey. They also had tentacles to help them catch prey. Technically they are carnivores but prey-size was so small they are often referred to as microcarnivores.
The Rugosa is one of the most well-known of corals for its fossilized character. It's almost always in a cast because of its dead body sinking into sand and being filled in with clay and other inorganic particles.